1. Field of the Invention
This invention relates to a soundproof heat shield member having a three-dimensional shape, and more particularly to a soundproof heat shield member suitable for an exhaust manifold.
2. Description of the Related Art
In an engine room of an automobile are arranged various electronic parts for conducting engine control, run control and the like in addition to an engine. These electronic parts are exposed to a higher temperature from the engine as a heat source, so that they are protected by arranging a heat shield member onto the heat source. That is, the heat shield member is arranged around an exhaust manifold of the engine at a given distance separated therefrom to insulate heat from the electronic parts. Furthermore, the exhaust manifold is generally a noise source, so that the heat shield member is required to have a soundproof function.
As the conventional soundproof heat shield member, there have been known a single metal plate, a laminate of plural metal plates, a laminate of plural metal plates sandwiching an asbestos sheet therebetween and the like. However, the conventional soundproof heat shield members do not yet develop satisfactory soundproof function. In the conventional soundproof heat shield member, noise becomes frequently high due to solid-borne sound from the exhaust manifold. Furthermore, a space between the exhaust manifold and the soundproof heat shield member serves as a resonator, in which noise from the exhaust manifold is repeatedly reflected in the space to amplify the noise and hence the noise level in the soundproof heat shield member rises. Moreover, when the asbestos sheet is sandwiched as a sound absorbing material between the metal plates, the outer surface of the metal plate reflects noise and hence the sufficient sound absorbing effect is not obtained.
On the other hand, JP-U-1-158513 discloses a cover for exhaust manifold covering the exhaust manifold. This cover comprises an inner plate facing the exhaust manifold, an outer plate located outside the inner plate and a sound absorbing member sandwiching between the inner plate and the outer plate, in which plural openings are formed in the inner plate. In the cover of such a construction, the noise from the exhaust manifold is absorbed by the sound absorbing member through the openings of the inner plate. However, there is no consideration that the noise is repeatedly reflected on the portion of the inner plate other than the openings to amplitude the noise. In this cover, therefore, the noise control effect is low, and particularly the effect of controlling high-frequency noise is very low.